The purpose of a vehicular transmission is to provide a neutral, at least one reverse and one or more forward driving ranges that impart power from an engine, and/or other power sources, to the drive members which deliver the tractive effort from the vehicle to the terrain over which the vehicle is being driven. As such, the drive members may be front wheels, rear wheels or a track, as required to provide the desired performance.
A series propulsion system is a system in which energy follows a path from an engine to an electric storage device and then to an electrical motor which applies power to rotate the drive members. There is no direct mechanical connection between the engine and the drive members in a series propulsion system.
Transmissions adapted to receive the output power from either an engine or an electric motor, or both, have heretofore relied largely on what has been designated as series, hybrid propulsion systems. Such systems are designed with auxiliary power units (APU's) of relatively low power for minimum emissions and best fuel economy. However, such combinations of small APU's and even large energy storage devices do not accommodate high-average power vehicles or address duty cycles that demand continuous, constant speed operation. Steep grades and sustained high-average cruising speeds at desired high efficiencies are not achievable with a typical, series, hybrid transmission configuration.
The challenge, therefore, is to provide a power system that will operate at high efficiencies over a wide variety of operating conditions. Desirable electric variable transmissions should leverage the benefits of a series, hybrid transmission for desirable low-average power duty cycles—i.e.: low speed start/stop duty cycles—as well as the benefits of a parallel hybrid transmission for high-average output power, high speed duty cycles. In a parallel arrangement the power supplied by the engine and the power supplied by the source of electrical energy are independently connected to the drive members.
Moreover, perfecting a concept wherein two modes, or two integrated power split gear trains, with either mode available for synchronous selection by the on-board computer to transmit power from the engine and/or the motor/generator to the output shaft results in a hybrid transmission having an extremely wide range of applications.
The desired beneficial results may be accomplished by the use of a variable, two-mode, input and compound split, parallel hybrid electromechanical transmission. Such a transmission utilizes an input member to receive power from the vehicle engine and a power output member to deliver power to drive the vehicle. First and second motor/generator power controllers are connected to an energy storage device, such as a batter pack, so that the energy storage devices can accept power from, and supply power to, the first and second motor/generators. A control unit regulates power flow among the energy storage devices and the motor/generators as well as between the first and second motor/generators.
A variable, two-mode, input-split, parallel, hybrid electro-mechanical transmission also employs at least one planetary gear set. The planetary gear set has an inner gear member and an outer gear member, each of which meshingly engages a plurality of planet gear members. The input member is operatively connected to one of the gear members in the planetary gear set, and means are provided operatively to connect the power output member to another of the gear members in the planetary gear set. One of the motor/generators is connected to the remaining gear member in the planetary gear set, and means are provided operatively to connect the other motor/generator to the output shaft.
Operation in the first or second mode may be selectively achieved by using torque-transmitting mechanisms. Heretofore, in one mode the output speed of the transmission is generally proportional to the speed of one motor/generator, and in the second mode the output speed of the transmission is generally proportional to the speed of both motor/generators.
In some embodiments of the variable, two-mode, input-split, parallel, hybrid electro-mechanical transmission, a second planetary gear set is employed. In addition, some embodiments may utilize three torque-transmitting mechanisms—two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine. In other embodiments, all three torque-transmitting mechanisms may be utilized to select the desired operational mode of the transmission.
As those skilled in the art will appreciate, a transmission system using a power split arrangement will receive power from two sources. Utilization of one or more planetary gear sets permits two or more gear trains, or modes, by which to deliver power from the input member of the transmission to the output member thereof.
U.S. Pat. No. 5,558,589 which issued on Sep. 24, 1996, to General Motors Corporation, as is hereby incorporated by reference, teaches a variable, two-mode, input-split, parallel, hybrid electromechanical transmission wherein a “mechanical point” exists in the first mode and two mechanical points exist in the second mode. U.S. Pat. No. 5,931,757 which issued on Aug. 3, 1999 to General Motors Corporation, and is hereby incorporated by reference, teaches a two-mode, compound-split, electro-mechanical transmission with one mechanical point in the first mode and two mechanical points in the second mode.
A mechanical point occurs when either of the motor/generators is stationary at any time during operation of the transmission in either the first or second mode. The lack of a mechanical point is a drawback inasmuch as the maximum mechanical efficiency in the transfer of power from the engine to the output occurs when one of the motor/generators is at a mechanical point—i.e.: stationary. In variable, two-mode, input-split, parallel, hybrid electromechanical transmissions, however, there is typically one point in the second mode at which one of the motor/generators is not rotating such that all the engine power is transferred mechanically to the output.
Additional increases in powertrain operating efficiencies may be achieved by providing a variable displacement internal combustion engine, which operates on the principle of cylinder deactivation. During operating conditions that require high output torque, every cylinder of a variable displacement engine is supplied with fuel and air such that the engine can sustain combustion and provide torque. During operating conditions at low speed, low load, and/or other inefficient conditions for a fully displaced engine, cylinders may be deactivated to improve efficiencies of the variable displacement engine. For example, in the operation of a vehicle equipped with a four-cylinder variable displacement engine, fuel economy will be improved if the engine is operated with only two cylinders during relatively low torque operating conditions by reducing throttling losses. Throttling losses, also known as pumping losses, are the extra work that the engine must perform to pump air from the relatively low pressure of an intake manifold, across intake and exhaust valves, and out to the atmosphere. The cylinders that are deactivated will disallow airflow through their respective intake and exhaust valves, thereby reducing pumping losses by forcing the internal combustion engine to operate at a higher intake manifold pressure. Since the deactivated cylinders do not allow air to flow, additional losses are avoided by operating the deactivated cylinders as “air springs”due to the compression and decompression of the air retained in each deactivated cylinder.